Validate that rules without toolchain context cannot create actions

In theory, [`RuleContext.getToolchainContext`](https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java;bpv=1;bpt=1;l=1120?ss=bazel&q=RuleContext&gsn=getToolchainContext&gs=KYTHE%3A%2F%2Fkythe%3A%2F%2Fgithub.com%2Fbazelbuild%2Fbazel%3Flang%3Djava%3Fpath%3Dcom.google.devtools.build.lib.analysis.RuleContext%230725ce045590b002a909a757696908b4d56d671f12829e58b90ee6c328d33a1f) will only return `null` if the rule is not generating actions (as set in [`RuleClass.Builder.useToolchainResolution(ToolchainResolutionMode.DISABLED)`](https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/packages/RuleClass.java;bpv=1;bpt=1;l=1623?ss=bazel&q=RuleClass&gsn=useToolchainResolution&gs=KYTHE%3A%2F%2Fkythe%3A%2F%2Fgithub.com%2Fbazelbuild%2Fbazel%3Flang%3Djava%3Fpath%3Dcom.google.devtools.build.lib.packages.RuleClass.Builder%23d3d2f24cd8e66dd2de269d7fd550f7ddeac599016d8ab4474f700c51b430fb1b)). However, we aren't currently validating this, and it's possible that legacy rules are violating this.

We should add explicit validation around this to ensure that any rule which generates actions has a toolchain context (which includes both the target and exec platforms for that configured target).

Start by reading RuleContext.getToolchainContext and RuleClass.Builder.useToolchainResolution(ToolchainResolutionMode.DISABLED), which define the relevant context and rule configuration. Add validation ensuring action-generating rules have both target and exec platforms in their toolchain context, and verify that rules without toolchain context are rejected.

Written by the indexing model from the issue text.